OBJECTIVE: To determine the prevalence of pulmonary arterial hypertension in asymptomatic patients with 2 types of extensive slow-flow vascular malformations: extensive venous malformations or Klippel-Trénaunay syndrome (KTS).DESIGN: Case-control.SETTING: Multidisciplinary center for vascular anomalies. Patients A consecutive sample of 32 patients with slow-flow vascular malformations of at least 15% of the body surface was identified retrospectively and matched by age and sex with 32 healthy controls. Interventions Standard 2-dimensional transthoracic Doppler echocardiography. Venous samples were obtained the same day that echocardiography was performed.MAIN OUTCOME MEASURES: Pulmonary artery systolic pressure (PASP) was determined. Levels of D-dimer, fibrinogen, and von Willebrand factor (vWF) in plasma were measured.RESULTS: Patients had a mean (SD) PASP that was significantly higher than that of healthy controls (42.16 [8.49] mm Hg in patients vs 27.69 [6.54] mm Hg in healthy controls; P < .001). No significant differences in PASP were found between patients with KTS and patients with venous malformations (P = .80). We observed significant differences in the mean (SD) levels of vWF between patients and healthy controls (124.41% [52.28%] in patients vs 92.69% [28.92%] in controls; P = .01) and also in levels of D-dimer (1032.99 [1367.0] ng/mL in patients vs 102.97 [29.39] ng/mL in healthy controls; P < .001). There was a moderate positive correlation between levels of vWF and levels of PASP (r = 0.42; P = .001) and a high positive correlation between D-dimer and PASP (r = 0.52; P < .001)CONCLUSIONS: The presence of pulmonary arterial hypertension in patients with extensive slow-flow vascular malformations is not an isolated feature but is relatively frequent. Levels of D-dimer correlate with PASP in these patients.